Add same process option for exec-env

[ricmatsui]

When running sops exec-env, the command is currently launched in a child process, signals are not forwarded, and the command continues to run even if the sops process is killed. This prevents exec-env from being usable as a wrapper for launching commands such as servers where signals and process exiting needs to be handled correctly, as mentioned in #840.

This PR adds an option to launch the command in the same process via the execve system call, which causes the command to run with the new environment in the same process, immediately exiting from sops. This means signals and process exiting are handled correctly since the command is in control.

The option has been added to exec-env only, so it does not fully address #840 and more may be needed to properly handle temporary files created with exec-file. The option is also incompatible with the background option.

Related #1209